Postgres Logical Replication - how to see what subscriber is doing with received data?

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From: Michael Jaskiewicz <[email protected]>
To: [email protected] <[email protected]>
Subject: Postgres Logical Replication - how to see what subscriber is doing with received data?
Date: Thu, 29 Aug 2024 01:58:06 +0000
Message-ID: <BN9PR03MB59965B5087688309C0D79DCEB7962@BN9PR03MB5996.namprd03.prod.outlook.com> (raw)

I've got two Postgres 13 databases on AWS RDS.

  *   One is a master, the other a slave using logical replication.
  *   Replication has fallen behind by about 350Gb.
  *   The slave was maxed out in terms of CPU for the past four days because of some jobs that were ongoing so I'm not sure what logical replication was able to replicate during that time.
  *   I killed those jobs and now CPU on the master and slave are both low.
  *   I look at the subscriber via `select * from pg_stat_subscription;` and see that latest_end_lsn is advancing albeit very slowly.
  *   The publisher says write/flush/replay lags are all 13 minutes behind but it's been like that for most of the day.
  *   I see no errors in the logs on either the publisher or subscriber outside of some simple SQL errors that users have been making.
  *   CloudWatch reports low CPU utilization, low I/O, and low network.

Is there anything I can do here? Previously I set wal_receiver_timeout timeout to 0 because I had replication issues, and that helped things. I wish I had some visibility here to get any kind of confidence that it's going to pull through, but other than these lsn values and database logs, I'm not sure what to check.

Sincerely,
mj

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